"It is fun to take photos."
が feels better than は here. If you were comparing other things that were fun, the specificity of は would make sense. But this is quite a general statement, so 写真を撮るのが楽しいです feels more natural
Hmm, my initial reaction was that this seemed backwards, but having thought about it, I get where you're coming from - the restriction of scope caused by は can sometimes serve to contrast the topic you are selecting from others.
With は it's sort of like "(Out of all the stuff that's going on right now), as for taking pictures, that's fun."
With が it's more like you're newly discovering the fun of taking pictures, and expressing that. You point directly at the activity of taking pictures going on right now rather than sort of having the frame zoom in on it, so it's less contrastive.
Yeah, I see where this is coming from but using は feels a lot more natural than が for me in this case. I'm not sure why. Possibly because は is better for introducing new topics, or maybe because it pairs better with the の? I think you have it switched in regards to which is more general. は is more general, が is more specific. I'd love to hear a native person's take on this. But if anything, using が makes it sound more like a comparison to me. (が makes it sound like this, rather than other things, is fun)